Data storage device and associated method for writing data to, and reading data from an unpatterned magnetic layer

ABSTRACT

A data storage device includes a unpatterned magnetic film having data regions in which to store data. A track is disposed in proximity to the magnetic film, such that the track selectively defines a shiftable magnetic domain wall. In order to select a data bit that is stored in one of the data regions of the magnetic film, a fringing field of the magnetic domain wall in the track is used to selectively change a direction of a magnetic moment in the data region.

PRIORITY CLAIM

The present application is a continuation application of, and claims thepriority of co-pending U.S. patent application Ser. No. 10/685,835,titled “System and Method for Storing Data in an Unpatterned, ContinuousMagnetic Layer,” filed on Oct. 14, 2003, which is assigned to the sameassignee as the present application, and which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention generally relates to memory storage systems, andparticularly to a memory storage system that uses the magnetic moment ofmagnetic domains to store data. Specifically, the present inventionrelates to a system that stores data in an unpatterned, continuousmagnetic layer.

BACKGROUND OF THE INVENTION

There is great interest today in magnetic random access memory (MRAM) asa high performance, non-volatile memory using magnetic tunnel junction(MTJ) memory elements. The magnetic tunneling junction comprises twolayers of ferromagnetic material separated by a thin insulatingmaterial. The direction of the magnetic moment of one of the layers isfixed, for example, by exchange bias with an antiferromagnetic materialas described in U.S. Pat. No. 5,650,958, or by forming this layer from aferromagnetic material with a large magnetic anisotropy, much higherthan that of the second ferromagnetic layer, as described in U.S. Pat.Nos. 5,801,984 and 5,936,293.

The magnetic moment of this first layer responds very little to themagnetic fields which are applied during the operation of the memorydevice. By contrast, in the second ferromagnetic layer, the magneticmoment direction is allowed to move in response to magnetic fields whichare applied during the operation of the memory device to set thedirection of the magnetic moment of this layer. This applied field isreferred to as the writing field or switching field.

It is advantageous if the magnetization of both the first and secondferromagnetic layers are largely homogeneous and aligned along oneparticular direction so that the magnetic moments form essentially asingle magnetic domain. Usually the magnetic moment will not behomogeneous but, especially at the edges of the device where thedemagnetization fields are largest, the magnetization may be directedaway from the preferred direction.

Nevertheless, the direction of the magnetic moment in the second layerwill largely either be parallel or anti-parallel to the first layer,thereby allowing the storage of data in the form of “ones” and “zeros”.The individual MTJs are generally formed by micro-lithographicallypatterning a continuous film comprised of the magnetic tunnel junctionstructure, and thereby defining an MTJ element with a particular shapeand area. To create the MRAM the MTJ elements are incorporated within anetwork of “write” and “bit” lines for the purpose of reading andwriting the MTJ devices as described in U.S. Pat. No. 5,640,343.

Electrical circuits for setting and interrogating the state of theindividual MTJ elements may be formed by conventional CMOS processes.During the fabrication of the MRAM chip the MTJ elements will generallybe fabricated by first depositing a continuous magnetic multi-layeredfilm comprised of the magnetic tunnel junction structure on top of thearray of write (or bit) lines which have been fabricated in a priorstep. The MTJ film is then patterned using an etching technique to formthe ultra-small tunneling junctions of a particular shape and size.

One of the major problems with MRAM using MJTs is scaling the MTJelements to very small sizes. The switching fields of these elementsbecome ever more sensitive to the detailed structure of the edges ofthese devices as the devices become smaller. Small variations in shapesand sizes of the MTJs and edge roughness of these devices cause largechanges in the magnitude of field required for writing the individualdevice. Moreover, the field required to change the state of the MTJmemory element increases as the size of these elements decreases.

The magnetostatic fields emanating from the magnetic poles at the edgesof the MTJ memory elements primarily determine the required magnitude ofthe writing or switching fields. The structure of the MTJ device can bemade more complex to alleviate the role of the demagnetizing fields, forexample, by forming one or both of the ferromagnetic layers comprisingthe MTJ from sandwiches of two or more ferromagnetic layers separated bythin non-ferromagnetic layers, as described in U.S. Pat. Nos. 5,841,692,6,153,320, and 6,166,948.

In conventional MRAM cross-point architectures, the state of the storageelements is changed by passing currents through parallel arrays of“write-line” and “bit-line” wires near the MTJ memory elements.Typically, local fields at individual MTJ storage elements are createdby passing electrical currents through two wires placed just above andjust below the MTJ element.

An MTJ element is placed at the “cross-point” of each of the write andbit line wires. The write-line and bit-line wires are typically arrangedto be orthogonal to one another with the MTJ element oriented with itseasy magnetic axis oriented along one of the wires, usually referred toas the bit-line, although the MTJ device may also be oriented at someother angle with respect to the direction of the wires. A series of MTJstorage elements is arranged along each of the write and bit wires.

One selected element, at the cross-point of one of the bit-lines and oneof the writelines, is written by passing currents simultaneously alongthese wires. Along these same wires there will be a number ofhalf-selected elements which are subjected to either the write-linefield or the bit-line field.

The maximum fields that can be generated by the currents passing alongthe write and bit-line wires is limited to about 100 Oe for reasonablecurrent densities (or perhaps twice this amount if the wires are cladwith a highly permeable soft ferromagnetic material such as permalloy).The maximum current is limited ultimately by electro-migration wherebyatoms in the wires can be moved by current passing through the wirewhich can eventually lead to failure of the wire due typically to localnecking of the wire which results in an increase in the local currentdensity leading to a runaway process.

Thus, in actual devices the current density must be below this limit,but, even under these circumstances, the current density will be limitedby other considerations, including requirements on power dissipation andthe size of power transistors on the chip. Constraints on these currentsthereby constrain the maximum field and so limit the smallest size ofthe MTJ elements.

The switching field can be decreased by reducing the net magnetic momentof the element, for example, by using magnetic material with smallermagnetization values or by using less magnetic material. Less magneticmaterial can be used by reducing the thickness of the magnetic layers inthe MTJ device. However, these devices then become susceptible tothermal upsets due to the super-paramagnetic effect. Consequently, theMTJ elements must have sufficient magnetic anisotropy that they arestable against thermal fluctuations at the operating temperature of theMRAM device and, particularly, when these devices are half-selectedduring writing of elements in the MRAM cross-point architecture.

What is therefore needed is an MRAM architecture in which the switchingof the memory elements is not determined by the shape and size of thememory elements, and in which much larger local magnetic fields can begenerated, allowing smaller magnetic elements with sufficient magneticstability against thermal fluctuations. The need for such a system hasheretofore remained unsatisfied.

SUMMARY OF THE INVENTION

The present invention satisfies this need, and presents a system and anassociated method (collectively referred to herein as “the system” or“the present system”) for storing digital information in an un-patternedmagnetic film, the data storage layer, in a solid state device with nomoving parts. The present system provides a global method for writing toand reading from an unpatterned magnetic film. Briefly, the presentsystem uses the inherent, natural properties of the domain walls inferromagnetic or ferrimagnetic materials to write data on anun-patterned magnetic film. Data is read from the unpatterned magneticfilm using magnetic tunneling junctions (MTJs). The magnetic film is thestorage layer. The magnetic film may be a single continuous layer withinwhich multiple data bits are stored or may be a series of discontinuoussections which are not contiguous with one another.

The magnetic film may be comprised of a single magnetic layer or may becomprised of multiple magnetic layers which may be separated bynon-ferromagnetic spacer layers. This film is comprised of magneticmaterials similar to those used in magnetic media in conventionalmagnetic hard disk drives. These magnetic materials have sufficientintrinsic crystalline magnetic anisotropy that magnetic regions or bitswritten into this layer are stable against thermal fluctuations. Thismeans that the magnetic fields required to change the orientation ofthese magnetic regions or bits will be much larger than can be providedby currents passing through near-by wires. These larger magnetic fields,however, can be achieved by using the domain wall fringing fieldgenerated at the boundary between two magnetic domain walls in magnetictracks comprised of magnetic wires located in proximity to the magneticfilm.

Associated with each domain wall are large magnetic fringing fields. Thedomain wall concentrates the change in magnetism from one direction toanother in a magnetic material in a very small space. Depending on thenature of the domain wall, very large dipolar fringing fields canemanate from the domain wall. This characteristic of magnetic domains isused to write to data storage regions in the magnetic film. When thedomain wall is moved close to the magnetic film, the data storage layer,the large fields of the domain wall change the direction of the magneticmoment in a localized storage region within the storage layer,effectively “writing” a bit to the storage layer.

An important characteristic of domain wall fringing fields is that theyare localized in small regions of space near the domain wall. Thus,domain wall fringing fields can provide highly localized and largemagnetic fields that can be manipulated in space by moving orcontrolling the position of the domain wall within a magnetic entitysuch as a magnetic track or wire.

The fields created by the domain wall are large enough to write tomagnetic layers with magnetic switching fields of several thousandoersteds. The magnitude of the fringing fields drops rapidly withdistance from the domain wall. Consequently, application of the domainwall fringing fields can be controlled in magnetic tracks or wireslocated in proximity to the magnetic film, by varying the distance ofthe wire from the material whose property is to be changed by the domainwall fringing field, and by moving the domain wall along the wire.

In the present system, the magnetic storage regions within the storagelayer are written by using the fringing fields from magnetic domainwalls in neighboring magnetic tracks comprised of magnetic wires. Thesewires are brought close to the magnetic storage layer where the magneticstorage regions are to be written. In the quiescent state, the domainsmay be parked away from the storage layer in the wire sufficiently farfrom the surface of the storage layer that the fringing field may notinfluence the state of the storage layer.

The domain walls are moved in this wire from one side of the magneticregion, across the magnetic region (and close to this region) to theother side of the wire for writing the magnetic region. The currentflowing through the wire controls the movement of the domain wall, andconsequently controls the writing of the magnetic regions within themagnetic storage layer.

In one embodiment, each magnetic region within the storage layer has anassociated magnetic wire which is connected to a transistor so that anindividual magnetic region is switched by passing a current through theassociated magnetic wire to move the corresponding domain wall and itsassociated magnetic fringing field. Consequently, the magnetic region iswritten. Depending on which direction the domain wall is moved (left toright or right to left) the magnetic region is written in one directionor the other.

In another embodiment the magnetic wires associated with each magneticstorage region are connected together in series such that domain wallsassociated with each magnetic region are moved together across thestorage region. By having magnetic wires above and below the storagelayer a selected magnetic region is written by a combination of domainwall fringing fields from the wires above and below the chosen magneticregion. The half-selected regions are not written.

The magnetic film or data storage layer can be continuous, or,alternatively, it can be comprised of smaller magnetic sections that arenot contiguous with one another. These smaller magnetic sections canvary in size, from sections that are so small that they contain only one(or a small number of data storage regions), or they can be large enoughto contain a series of magnetic data storage regions associated with oneor more writing tracks.

In a preferred embodiment, the magnetic section is sufficiently largesuch that the data storage region (or regions) does not extend to theedges or the boundary of the magnetic section, so that the detailedmorphology and shape of the edges of the section do not influence themagnitude of the magnetic fields required to write the magnetic regionor regions within the section. As a result, the individual data storageregions are not “patterned” into special areas with particular shapes asin conventional MRAM architectures, but they rather reside in an“un-patterned” magnetic film or data storage layer. Similarly, the datastorage regions are not localized in small nano-patterned magneticelements of a particular shape and size, but they are rather simplyregions within an un-patterned, and otherwise homogeneous magnetic film.The locations of the magnetic regions within the magnetic film areconveniently determined by the writing element.

One advantage of this invention compared to conventional MRAM designs isthat the field required to write the magnetic regions in the unpatternedmagnetic data storage layer is influenced only by the magneticproperties of this layer and not by the detailed physical structure of anano-patterned magnetic element. Furthermore, the use of domain wallfringing fields in magnetic writing elements may allow much higher localmagnetic fields than are possible to generate from electrical currentspassing through non-magnetic wires. This allows for an MRAM that canachieve much higher densities than is possible with conventionalcross-point designs.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features of the present invention and the manner ofattaining them will be described in greater detail with reference to thefollowing description, claims, and drawings, wherein reference numeralsare reused, where appropriate, to indicate a correspondence between thereferenced items, and wherein:

FIG. 1 is comprised of FIGS. 1A, 1B, 1C, 1D, and 1E, and illustratesvarious embodiments of a magnetic memory device according to the presentinvention;

FIG. 2 is comprised of FIGS. 2A, 2B, and 2C, and schematicallyillustrates expanded views of one data bit and its corresponding writingand reading structure as utilized by the magnetic memory device of FIG.1;

FIG. 3 is comprised of FIGS. 3A, 3B, and 3C, and represents a schematicdiagram and two graphs illustrating domain wall fringing fields of themagnetic memory device of FIG. 1;

FIG. 4 is a process flow chart illustrating a method of operation ofwriting to the magnetic memory device of FIG. 1;

FIG. 5 is comprised of FIGS. 5A, 5B, 5C, 5D, 5E, and 5F, and illustratesthe process of writing to the magnetic memory device of FIG. 1;

FIG. 6 is comprised of FIGS. 6A and 6B, and represents a configurationof an alternative embodiment of the writing device of the magneticmemory device of FIG. 1, using a block of ferromagnetic or ferrimagneticmaterial;

FIG. 7 represents a configuration of an alternative embodiment of thewriting device of the magnetic memory device of FIG. 1, using multipletypes of ferromagnetic or ferrimagnetic material;

FIG. 8 represents a configuration of another alternative embodiment ofthe writing device of the magnetic memory device of FIG. 1, usingindentations (or alternatively protuberances) in a track offerromagnetic or ferrimagnetic material;

FIG. 9 is a process flow chart illustrating a method of operation ofreading data from the magnetic memory device of FIG. 1;

FIG. 10 is comprised of FIGS. 10A and 10B, and illustrates the processof reading data from the magnetic memory device of FIG. 1;

FIG. 11 is comprised of FIGS. 11A, 11B, and 11C, and illustratesdifferent materials that can be used in the magnetic film of themagnetic memory device of FIG. 1;

FIG. 12 is a diagram illustrating the use of a reservoir for domains inthe writing and reading tracks of the magnetic memory device of FIG. 1;

FIG. 13 is comprised of FIGS. 13A and 13B, and illustrates analternative embodiment of the magnetic memory device of FIG. 1 usingindividually controlled registers to write to the magnetic film;

FIG. 14 is comprised of FIGS. 14A and 14B, and illustrates an embodimentof the magnetic memory device of FIG. 13; and

FIG. 15 is comprised of FIGS. 15A and 15B, and illustrates anotherembodiment of the magnetic memory device of FIG. 13.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Conventional MRAM architectures involve the use of discrete magneticmemory elements with a particular shape and size that arelithographically patterned by, for example, conventional optical and/orelectron beam lithography and etching. Typically, these memory elementsare fabricated on top of micro-electronic components, such astransistors and capacitors, which form circuits which are used for thepurposes of reading, writing and interrogating the magnetic memoryelements.

These components are formed first, typically on silicon wafers, and thenthe magnetic memory elements are fabricated, usually by first depositinga continuous film across the wafer which comprises the magnetic memoryelement. The film is then lithographically patterned into magneticmemory devices of a particular and well defined shape and size which arealigned very precisely with wires or vias which connect these elementsto the underlying electronic components.

Finally, vias and wires are fabricated on top of the patterned magneticmemory elements to provide the final electrical connections to theseelements. Typically, the performance of the magnetic memory is verysensitive to the detailed shape and size of the magnetic memory elementsand to their precise placement with respect to the circuitry above andbelow these elements. Thus, the shape and size of the elements must bevery well controlled. This becomes increasingly difficult as the size ofthe elements is shrunk to allow for ever high memory storage capacities.

FIG. 1A illustrates an exemplary high-level architecture of a magneticmemory device 100 comprising an unpatterned, continuous magnetic film10A. The magnetic data storage layer 10A is unstructured and does notspecify the location of each data bit, allowing very dense writing ofdata to this magnetic film.

As used herein, the term “patterned” indicates that the storage layer iscomprised of a series of sections, each of which is sufficiently largeto accommodate at least one data bit that comprises a magnetic regionwithin the magnetic storage layer. In addition, each section issufficiently large that the edges of this section do not significantlyinfluence the writing of data bits to be stored therein.

The term “un-patterned” generally indicates that the storage layer iscontiguous over the array of storage data bits. Patterned regions aresections that can be of any desirable shape, such as circular,elliptical, rectangular, square, or trapezoidal. Patterned sections canalso be large enough to comprise one or more rows or columns of databits.

The magnetic memory device 100 utilizes a series of writing devices(also referred to herein as writing element) 20. The writing device 20is comprised of many tracks made of individual magnetic wires,represented by magnetic tracks such as tracks 25, 30, 35, 40. The tracksare comprised of magnetic domains wherein the magnetization alternatesfrom one direction of magnetization to the opposite direction preferablyalong the tracks from one domain to the next. For purposes ofillustration only, the magnetic domains in neighboring tracks are shownto be oppositely magnetized in FIG. 1A but in operation the orientationof magnetic domains from one track to the next track need not becorrelated and may vary depending on the history of the writing element.

In FIG. 1A, the writing device 20 is situated both above and below theplane of the magnetic film 10A. For simplicity of reference, exemplarytracks 25 and 30 are also referred to herein as upper tracks becausethey are located above the plane of the magnetic film 10A. Similarly,exemplary tracks 35 and 40 are also referred to herein as lower tracksbecause they are located below the plane of the magnetic film 10A. Itshould be clear that the terms “upper” and “lower” are not used hereinas limiting terms but rather for illustration purposes only.

In this exemplary embodiment, tracks 25, 30, 35, and 40 are formed suchthat arch-like bends in the wire almost touch the magnetic film 10A.Data is written into each data bit represented by data bit 45 where thearch-like bend of a track above the magnetic film coincides with anarch-like bend in a track below the magnetic film.

The magnetic film 10A is coated with a thin insulating layer 15A on itstop surface and a generally similar, thin insulating layer on its bottomsurface. Preferably one or otherwise both of these insulating layers mayform the tunnel barrier component of a magnetic tunnel junction for thepurpose of reading the state of the data bits in the layer 10A.

An insulating material surrounds the magnetic film 10A and the writingelement 20, and is made during the process of fabricating the magneticfilm 10 and the writing element 20, to form a solid state memory device.

When a current is passed through a ferromagnetic metal, the current,which is comprised of spin-up and spin-down electrons, becomesspin-polarized because the electrical conductivity of the spin-up andspin-down electrons can be quite different in magnetic metals. When spinpolarized electrons (which, by definition travel in the oppositedirection to the current) are passed from a first side of a magneticdomain wall to its other side, the current becomes spin polarized in adirection along the direction of magnetization on the first side of thedomain wall.

The spin polarized electrons then deliver spin-angular momentum to theoppositely magnetized material on the second side of the wall whichcauses the magnetic moment on the second side of the wall to rotate,thereby inducing the magnetic domain wall to move in the direction ofthe spin polarized electrons, i.e., in the opposite direction to thecurrent direction, for majority spin-polarized electrons. When thecurrent is passed in the reverse direction the domain walls will alsomove in the opposite direction.

As current is passed along the tracks (i.e., track 25 and track 35) ofthe writing device 20, the domains in the tracks are moved to the leftor to the right, depending on the direction of the current. As thedomains move, the fringing fields from the domain walls write to themagnetic film 10A. Along the tracks, each of the fringing fieldsassociated with each domain wall may be writing. However, the field ofthe fringing fields is chosen such that data is not written to a databit 45 unless the fringing field in the upper track 25 is combined withthe fringing field in the lower track 35 applied to the same data bit45.

In FIG. 1A, the tracks are shown with a series of domain walls regularlyspaced along the tracks with the domain walls formed at the boundariesbetween regions of the tracks magnetized in opposite directions alongthe track. It should be understood that the domain walls do notnecessarily need to be regularly spaced apart. In FIG. 1A, the domainwalls are shown in their quiescent state so that they are not writing tothe magnetic storage film.

The operation of the writing device 20 is illustrated by the detaileddrawings in FIG. 2 (FIGS. 2A, 2B, 2C). The writing device 20 iscomprised of ferromagnetic tracks or wires such as wire 25 and wire 35that are placed in proximity to the magnetic film 10A. The wire 25 andwire 35 are brought in closer proximity to the film 10A by the arch-likebends in the wires.

As illustrated for exemplary purposes, the ferromagnetic track 25 iscomprised of a plurality of successive domains, such as exemplarydomains 205 and 210, magnetized in opposite directions. Ferromagnetictrack 35 is comprised of a plurality of successive domains 215, 220,also magnetized in opposite directions along the track. These magneticdomains, 205 and 210, and, 215 and 220, define domain walls 225 and 230,there between.

The magnetization directions of the domains 205 and 210, and, 215 and220 are set during an initialization of the memory device. Theinitialization stage needs to be carried out only once, after the memoryhas been fabricated and before the operation of the memory device. Thisis accomplished by the detailed design of the magnetic tracks and by,for example, applying large external magnetic fields in the propersequence.

The initial state of the tracks can also be established by injectingmagnetic domains from the ends of the tracks, for example, from magneticregions whose magnetization can readily be rotated from one direction tothe other in magnetic fields smaller than those which are required tochange the magnetic state of the tracks themselves. These magneticregions may, for example, be wider than the tracks since the magneticswitching field of narrow magnetic wires or regions decreasesapproximately inversely with their width.

Once a magnetic domain is injected into the wire the domain may be movedalong a track or wire by passing a current pulse into the wire. By usinga sequence of domain wall injection and current pulse induced domainwall motion, a sequence of domain walls can be established along eachtrack. Once the magnetic domains have been set up in the tracks, theyare subsequently moved only by passing currents through the tracks forthe purposes of writing data bits to the magnetic storage film.

With specific reference to FIG. 2B, a first domain wall 225 isassociated with fringing field 235, and a second domain wall 230 isassociated with fringing field 240. When both of the domain walls 225and 230 of the writing device 20 pass the region (also referred to asdata region) closest to the magnetic film 10A, the large magnetic fieldemanating from the domain wall 225 in conjunction with that emanatingfrom the magnetic domain wall 230 of the writing device 20 are combinedto enable the writing of a data bit, in the data region, within themagnetic storage film 10A.

To write to the magnetic storage film 10A, the writing device 20selectively changes the direction of the magnetic moment of a data bit45 in the magnetic film 10A. In the present illustration, the magnitudesof the fringing fields 235 and 240, applied to the magnetic storage film10A, decrease rapidly outside the region around the respective domainwalls 225 and 230, either in the writing device 20 or the magnetic film10A.

Thus, the magnetic fringing field associated with the domain walls 225and 230 can be used to provide a highly localized and large magneticfield to the magnetic storage film 10A, allowing the writing of a smallmagnetic region 45 within the magnetic film 10A. The magnitude of thefringing field 235 and 240 applied by the writing device 20 to themagnetic film 10A can be controlled by controlling the relative locationof the domain walls 225 and 230 in the writing device 20.

In addition, the location of the domain wall 225 is controlled byapplying a current 250 along track 25. Similarly, the location of thedomain wall 230 is controlled by applying a current 255 along track 35.

As shown in FIG. 2C, the domain walls 225 and 230 are moved away fromthe magnetic bit 45 by currents 250 and 255, respectively, after the bitis written. The tracks 25 and 35, as shown in FIGS. 2A and 2C, havenotches on either side of the arch shaped regions in these tracks forthe purposes of fixing the position of the domain walls when they arenot being used for writing.

During the write operation, current pulses, such as pulses 250 and 255,are applied to the respective tracks 25 and 35 of sufficient magnitudeand temporal duration that the corresponding domain walls 235 and 240are moved from one notched position on one side of the arched shapedregion to a second notched position on the other side of the archedshaped region.

During the motion of the domain walls between the notched positions thedomain walls and their associated magnetic fringing fields are broughtclose to the magnetic storage film so that the magnetic storage film issubjected to a large and localized magnetic field sufficient to set themagnetization direction of the small magnetic region in one direction orthe opposite direction and so write a magnetic bit.

In FIG. 2, the writing of the magnetic region 45 in the magnetic film10A is accomplished by the combination of the magnetic fringing fieldsfrom the domain wall in the track 25 above the storage film 10A and thatfrom the track 35 below the storage layer 10A.

In another embodiment of this invention writing is accomplished by themagnetic fringing field from only one domain wall from a single trackeither above or below the magnetic storage film 10A.

In FIG. 1A, the magnetic film 10A is shown as a continuous andun-patterned magnetic film. However, in other embodiments, illustrated,for example, in FIGS. 1B through 1E, the magnetic films 10B through 10E,respectively, are comprised of multiple sections of magnetic materialeach distinct from one another.

For example, as illustrated in FIG. 1B, the magnetic film 10B iscomprised of a series of approximately circularly (or elliptically)shaped sections, i.e., 101, 102, 103, wherein a single data bit may beselectively written. These sections 101, 102, 103, are distributedalong, and aligned with, each “column” and “row” of writing deviceswithin the writing element 20.

These sections 101, 102, 103 are sufficiently large to contain only onemagnetic bit so that each data bit shown in FIG. 1A is contained withinone section of the magnetic film. The magnetic bit does not extend tothe very edges of the magnetic sections, so that the writing fieldsrequired to write the bit are not affected by the detailed shape andmorphology of the edges of the sections 101, 102, 103. Thus, themagnetic sections 101, 102, 103 do not need to be precisely aligned withthe corresponding upper and lower magnetic tracks associated with eachsection, making the fabrication of the memory device much simpler,easier, and less expensive than comparable devices.

In the embodiment illustrated herein, a series of insulating layers 15B,correspond in shape to, and are formed on, the sections 101, 102, 103.However, the layer 15B may also extend beyond the edges of the magneticsections 101, 102, 103, and may also be continuous, extending from onesection to another section.

This extent of the layer 15B depends on the detailed method offabrication of the magnetic memory device 100. Furthermore, the layer15B does not need to be precisely aligned with the magnetic filmsections 101, 102, 103. The layer 15B needs only cover the middleportion of the magnetic sections 101, 102, 103 that touch the uppermagnetic tracks 25, 30, to form a tunneling barrier for readingpurposes.

A similar insulating layer may optionally coats the underside of thesections 101, 102, 103. This optional insulating layer may also form atunneling barrier layer within a magnetic tunnel junction readingdevice. However, because the sections 101, 102, 103 contain a singledata bit and intersect with only one upper track and one lower track,the lower track may be in electrical contact with the magnetic sectionand no isolation layer between the magnetic section and the lower trackis needed.

FIG. 1C illustrates another embodiment of the writing device 20, whereinthe magnetic film 10C is comprised of a series of approximatelyrectangularly (alternatively square, triangular, or any other suitable)shaped sections, i.e., 104, 105, 106, wherein a single data bit may beselectively written. These sections 104, 105, 106 are distributed alongand aligned with each “column” and “row” of writing devices within thewriting element 20. In this embodiment, a series of insulating layers15C, correspond in shape to, and are formed on, the sections 104, 105,106.

Similarly to the layout described earlier in connection with FIG. 1B,the layer 15C may be limited in extent to a region where the uppertracks 25, 30 come in proximity to the magnetic sections, 104, 105, 106.Alternatively, the layer 15C may be contiguous between magneticsections. A corresponding thin insulating layer may, optionally, bedisposed on the underside of the magnetic sections 104, 105, 106.

FIG. 1D illustrates another embodiment of the writing device 20, whereinthe magnetic film 10D is comprised of a series of columns, i.e., 107,108, 109. In this embodiment, a series of insulating layers 15D,correspond in shape to, and are formed on, the sections 107, 108, 109.

FIG. 1E illustrates another embodiment of the writing device 20, whereinthe magnetic film 10E is comprised of a series of columns, i.e., 110,111, 112. In this embodiment, a series of insulating layers 15E,correspond in shape to, and are formed on, the sections 110, 111, 112.

In both the embodiments shown in FIGS. 1D and 1E the correspondinginsulating layers 15D and 15E may, on the one hand, be limited inlateral extent to the region of close proximity of the upper tracks andthe magnetic film or may, on the other hand, be contiguous betweenneighboring tracks. Similarly, there may or may not be a thin insulatinglayer on the lower surface of the corresponding magnetic sections.

Regardless of how the magnetic film 10 is comprised of smaller magneticsections, the sections are sufficiently large that they behave like anun-patterned magnetic film to the individual magnetic bits. Thus, theperipheral region of the magnetic section, i.e., the detailed shape andmorphology of the edges of the section, do not influence the writing ofthe individual magnetic bits, contrary to the case of conventional MRAMswith individual patterned magnetic storage bits.

FIG. 3 (FIGS. 3A, 3B, 3C) illustrates the concept of domains, domainwalls, and fringing fields, as used in conjunction with the presentinvention. FIG. 3A shows an exemplary layer of a magnetic material, withwhich the tracks of the writing element 20 are comprised, with twomagnetic domains 305 and 310 with their magnetizations directed alongopposite directions along the x axis.

The arrows, such as arrow 315, represent a magnetic moment, or dipole,and indicate local magnetization directions. The magnetic moments indomain 305 point to the right, while the magnetic moments in domain 310point to the left. The domain wall 320 is the region in which domains310, 305 of opposite polarity meet. The change of magnetization betweendomain 305 and domain 310 is concentrated in the small region or domainwall 320, creating a large dipolar fringing field emanating from thesurface of the layer.

The relative magnitude of an exemplary fringing field B is shown in FIG.3B. The fringing field B is localized and concentrated over a region of,for example approximately 100 nm, in the x dimension. The peak values ofthe components Bx, By, and Bz of the fringing field B, are illustratedin FIG. 3C as a function of out-of-plane distance. The fringing field isalso localized in the z direction, and is concentrated primarily in aregion of approximately 20 nm in the z direction.

These fringing field components Bx, By, and Bz are very high in theregion of the domain wall 320, and drop off rapidly with distance fromthe domain wall 320. Consequently, the fringing field B is localized andsufficiently large for use to magnetize a second magnetic material in asmall localized region.

The detailed magnitude and spatial variation of the components of thefringing field B depend on the details of the spatial variation of themagnetization in the domain wall 320. The distribution of themagnetization within the domain wall depends on the magnetic parametersof the magnetic material in the magnetic layer comprising the magnetictracks of the writing element 20, particularly, the magnetization(magnetic moment per unit volume), the magnetic anisotropy and thestrength of the magnetic exchange, as well as the thickness and width ofthe magnetic layer.

The fringing field B is used to write magnetic regions within themagnetic film 10. When the domain wall 320 is moved close to anothermagnetic material, the large fringing field B of the domain wall 320changes the direction of the magnetic moment in this second magneticmaterial, effectively “writing” to this magnetic material. This domainwall 320 can be moved within the magnetic wire of the magnetic track ofthe writing element 20 by passing a current through the magneticmaterial of the magnetic wire that is perpendicular to the domain wall320.

The magnetic moment direction of the domains can be either along thedirection of the magnetic track or perpendicular to the direction of themagnetic track of the writing element 20. For very narrow tracks, thepreferred direction of the magnetic moments are along the trackdirection.

A method 400 of the writing device for writing data according to thepresent invention, is illustrated by the process flow chart of FIG. 4,considered in conjunction with FIG. 5 (FIGS. 5A, 5B, 5C). The domainwall and its associated magnetic fringe fields are shown in a quiescentposition within the magnetic track outside a write region 505 (FIG. 5A)of the writing device 20 in FIG. 5. A request to write data is receivedby the memory system.

At block 405 of FIG. 4, the memory system translates the data (0 or 1)into whether the data bit 510 in the magnetic data storage film willreceive a magnetic moment pointing right (a right magnetic moment) or amagnetic moment pointing left (a left magnetic moment). If, at decisionblock 410, the data bit 510 is to be written with a left magneticmoment, method 400 proceeds to block 415.

At block 415, a current 515 is applied to the track 520 of the writingdevice 20 in FIG. 5A, moving the domain wall 525 in the positivedirection (block 420), as noted by the directional arrow of current 515.Track 520 is located above the magnetic film 10. In response to theapplication of current 515, fringing field 530 is moved to within thewrite region 505 (FIG. 5C).

FIG. 5B shows the domain wall 525 at an intermediate position betweenthe quiescent position in FIG. 5A and the writing position in FIG. 5C.Concurrently and in a similar manner, a current 545 is applied to atrack 540 located below the magnetic film 10, oriented perpendicular totrack 520. The end of the arrow for current 545 shows that the currentis flowing in the direction into FIG. 5A, 5B, 5C.

At data bit 510, and only at data bit 510, track 520 and track 540 areboth in close proximity to the data film 10. A current 545 is applied totrack 540, moving the fringing field 550 within the write region 505.The magnetic fringe fields 530 and 550 write to the data bit 510 (block425), changing the direction of the magnetization of data bit 510 topoint in the desired direction.

The currents 515 and 545 are then applied to tracks 520 and 540 (block430) so that the fringing fields 530 and 550 are removed from the regionof the data bit 510. The fringing fields 530, 550 may be returned totheir prior position or moved further down their respective tracks 520,540 (block 435).

The writing device fringing fields 530 and 550 remain in close proximityof the magnetic film 10 for only an instant, or a predetermined periodof time, that is sufficient to write to the data bit 510 of the magneticfilm 10. The magnitude of fringing fields 530 and 550 that is applied todata bit 510 is large only when the domain walls tracks 520 and 540 arein proximity to the magnetic storage film 10.

If the data bit 510 is to be written with a right magnetic moment atdecision block 410, method 400 proceeds to block 440. At block 440, acurrent 515 is applied to the track 520, as illustrated in FIG. 5D, tocause the domain wall 555 to move in the negative direction (block 445).

A fringing field 560 is shifted within the write region 505 (FIG. 5D).Concurrently and in a similar manner, a current 545 is applied to atrack 540 located below the magnetic film 10, oriented perpendicular totrack 520. At data bit 510 and only at data bit 510, track 520 and track540 are both in close proximity to the data film 10.

A current 545 is applied to track 540, moving the fringing field 565within the write region 505. The tip of the current 545 shows that thecurrent is flowing out of FIG. 5D, 5E, 5F. The magnetic fringe fields560 and 565 write to the magnetic film 10 (block 450), changing thedirection of the magnetization of data bit 510 to point in the rightdirection. Currents 515 and 545 are then applied to tracks 520 and 540,respectively, and the domains are moved away from the data bit 510(block 435 of FIG. 4).

In another embodiment of the current invention the tracks 520 and 540 inFIG. 5 are fabricated as straight wires without any arch-like bends.This means that the magnetic storage film will always be subjected tolarge domain wall fringing fields even when the domain walls are intheir quiescent position.

In the embodiment shown in FIG. 5, in which magnetic regions are writtenby a combination of magnetic fringing fields from domains in two tracks,this is acceptable although less desirable than in the embodiment shownin FIG. 5, in which the arch-like bends prevent large fringing fieldsfrom impinging on the storage film except during the writing process.

The properties of the magnetic storage film 10 and the writing device 20are chosen so that the fringing field from a single domain wall is notsufficient to change the magnetization direction of the magnetizationwithin the magnetic storage film.

In another embodiment, shown in FIG. 6 (FIGS. 6A, 6B), the arch-shapedbends in each track 605, 610 are replaced with small ferromagneticblocks 615 and 620. A thin layer 625, 630 is formed on top of theferromagnetic blocks 615, 620.

The thin layer is used to determine the spacing of the ferromagneticblocks 615, 620 from the magnetic film 10. Since the writing deviceshould not preferably be in electrical contact with the magnetic film10, (except possibly when the magnetic film is divided into sectionscorresponding to one magnetic bit, as shown in FIGS. 1B and 1C) the thinlayer 625, 630 may be formed from an insulating material that may becontiguous with the insulating material that surrounds the writingdevice 20B and the magnetic film 10.

The domain wall 635 is pushed to the middle of ferromagnetic block 615by the current 640. The embodiment shown in FIG. 6 may be easier tofabricate than the embodiment shown in FIG. 5.

In a further embodiment, the homogeneous ferromagnetic or ferrimagneticmaterial in the writing device 20C can be replaced by inhomogeneousferromagnetic or ferromagnetic material, as shown in FIG. 7. Track 705of the writing device 20C is constructed of alternating types offerromagnetic or ferrimagnetic materials.

For example, blocks 710 and 720 are formed from one type of magneticmaterial while blocks 715 and 725 are formed of another type. Thesealternating types of ferromagnetic or ferrimagnetic materials serve tocreate defined regions in which the domains in the track 705 reside intheir quiescent position. The magnetic moments of blocks 710 and 720 arealigned in the same direction, so that no fringing fields are applied tothe magnetic film 10 when the writing device 20C is in the quiescentstate.

Writing to the magnetic film 10 is carried out by moving the domainwalls along the magnetic track 705 by passing a current along thistrack. When the domain walls 730, 735 are moved, by application of acurrent pulse (or a train of pulses) in the proper direction along thetrack 705, the corresponding domain wall 730 or 735 is brought to thewriting region 740, where the track is in close proximity to themagnetic film 10. The track may be designed, for example, by having adifferent width in the proximity of the data region 740, so that thedomain wall preferably resides in this position and requires a secondcurrent pulse to move the domain wall from the writing position to itsquiescent position, away from the writing region.

Although only the upper track of the writing device 20C is shown in FIG.7, a similar lower track may also be added. This lower track may bepositioned at an angle to the upper track. The writing of the dataregion 740 is only carried out when this region is subjected to acombination of magnetic fields from domain walls moved to the writingregion in both the upper and lower tracks.

It may be preferred that the domain wall in either the upper or lowertracks is first moved to the writing region 740, so that the region 740is subjected to a first magnetic field. Subsequently, the second domainwall is moved to the opposite side of the writing region, or is movedacross the writing region 740, without stopping. This method may beadvantageous so that the timing of the movement of the domain walls inthe corresponding upper and lower tracks is not so critical, as whenboth the domain walls in the upper and lower tracks are movedconcurrently through the magnetic writing region, without eitherstopping in this region.

In one embodiment, the domain walls are moved so that they are situatedover the data bits 740, 745 for a time determined by sequential currentpulses. The first pulse moves the domain wall fringing fields 730 and735 near the magnetic film 10. In a similar manner, a current pulsemoves domain walls along a corresponding track below the magnetic film10 so that the fringing field from the domain wall below the magneticfilm 10 arrives concurrently with that from above the layer 10.

By adjusting the time of arrival of the domain walls and theirassociated fringing fields, the length of time for which the combinedfield from the domain walls is large enough to write the magnetic layer10 can be adjusted. It may be advantageous to have different strengthsof fringing fields from the two domain walls above and below the layer10. The domain wall fringing field can be readily varied either byvarying the physical separation of the tracks from the layer 10 or byvarying the properties of the magnetic material from which the tracksare fabricated or by varying the shape and size (e.g., thickness andwidth) of the tracks.

In another embodiment, the magnetic regions or domains 710 and 720 canbe combined and formed from one magnetic material. The magnetic regions715 and 725 together are formed from an alternate magnetic material.This pattern may be repeated throughout the track 705. There is no needto have a means of providing a pinning potential for the domain walldirectly above the magnetic film 10. The writing of the magnetic film 10can be performed by simply passing the domain wall and its associatedfringing field near the magnetic film 10 without stopping the domainwall above the bit to be written.

In general, the track used to provide the domain walls for writing themagnetic bits into the magnetic storage film 10 is designed so that thedomain walls reside at well defined positions away from the writingpositions in their quiescent state. The positions are defined either byshaping the wire, for example, by forming notches, where the track widthis slightly reduced, or, alternatively, by protuberances, where thetrack width is increased slightly, or by forming the track from regionsof different magnetic materials with different magnetic properties, forexample, different magnetizations and/or magnetic anisotropies or bycombinations of these methods.

The embodiment in FIG. 7 shows the use of both notches and alternatingmagnetic materials to define the positions of the domain walls in thequiescent state. The choice of which method is used may dependparticularly on the method and ease of fabrication of the tracks.

In addition to fabricating the track so that the domain walls reside atparticular places along the track on either side of the writingpositions, the track can also be designed, by using the placement ofmagnetic materials or by shaping the track, so that the domain wall willalso reside at the writing position. In the latter case, one currentpulse is used to drive the domain wall from one of the quiescent statepositions to the writing position and a second current pulse is used todrive the domain wall from the writing position to one of the quiescentstate positions (this may be the same quiescent position from which thedomain wall was first moved or this may be the quiescent position on theother side of the writing position).

Furthermore, the tracks above and below the magnetic storage film can bedesigned differently so that one track may have a fixed position for thedomain wall during writing and the other track may not have such aposition. Thus, for the purposes of writing a magnetic region into themagnetic film 10 a current pulse may first be used in one of the tracksto drive a domain wall to the writing position in this track.

Then, a current pulse in the corresponding track on the other side ofthe magnetic film may be used to drive a domain wall across the writingposition from one quiescent state to the other. Thus, the magnetic bitis written by the combination of a static field provided by one domainwall fringing field and a dynamic, or moving domain wall fringing fieldprovided by the domain wall in the opposing track. As a result, methodsof precessional rotation of the magnetization of the magnetic bit 45 canbe used by first applying, with the domain wall fringing fields from onetrack, a static magnetic field in one direction and then, using thedomain wall fringing fields from the second track, a dynamic magneticfield in the perpendicular direction.

The detailed spatial variation of the domain wall fringing fields willdepend on the structure of the magnetic domain wall which, in turn, willdepend on the magnetic properties of the magnetic materials forming thetrack as well as the detailed shape and size of the track, i.e.,thickness and width of the magnetic material. The spatial variation ofthe fringing fields can be determined by micromagnetic simulations ofthe magnetic structure of the track.

These simulations show that the magnetization variation within thedomain wall can be complex and can vary significantly as, for example,the thickness and width of the track is varied. Thus, the spatialdistribution of the domain wall fringing fields can be optimized bymicromagnetic modeling.

In particular, depending on these details, the sign of the domain wallfringing fields may depend on whether the domain wall is moving or isstationary and may depend on the direction of motion of the domain wall.Thus, the detailed procedure for writing the magnetic regions into themagnetic storage film 10 will depend on the detailed magnetic structureof the domain wall and its associated fringing fields.

The detailed structure and composition of the tracks which comprise thewriting device 20 will influence the distribution of magnetization inthe domain walls and thus the domain wall fringing field distribution.Thus, the direction of the magnetization of the written bit 45 willdepend on the detailed structure of the writing device 20. Therefore,the direction of the magnetization of the bit 45 can be designed to belargely along one of the tracks in the writing device 20, e.g., thealong the direction of the upper tracks or the direction of the lowertracks, or can be designed to lie along a direction at some angle to theupper or lower track direction.

The magnetic material forming the tracks can be comprised of variousmagnetic materials such as Fe, Co, and Ni, and binary and ternary alloysformed from a combination of these ferromagnetic metals, or from alloysformed from one or more of these elements in combination with otherelements chosen, for example, for the purposes of providing highermagnetic anisotropy. Since the magnitude of the fringing fields from thedomain wall depends on the width of the domain wall and typicallyincreases as the domain wall is narrowed, it may be advantageous to formthe tracks from magnetic material with higher magnetic anisotropy thancan be obtained from alloys of Fe, Co and Ni alone. For example, themagnetic anisotropy of Fe and Co and Ni can be increased by combiningthese elements with Pt or Pd.

The domain wall fringing fields will also be increased, when themagnetization of the magnetic material is higher, so it may beadvantageous to form the track from alloys containing large amounts ofFe and Co, which have higher magnetization values than Ni. The choice ofthe material forming the track will also influence the magnitude of thecurrent and the length of the current pulse required to move the domainwalls along the tracks.

The higher the magnetic anisotropy, the larger the current pulsemagnitude and length are likely to be. Similarly, the damping factor ofthe magnetic material is important in influencing the magnitude of thecurrent pulse required to move the domain walls. The damping factor canbe varied by doping the magnetic material with transition or rare-earthmetals as, for example, described in U.S. Pat. No. 6,452,240.

An important advantage of using current pulses to change the directionof the magnetization of the track by moving domain walls thereby, isthat current pulses of reasonable magnitude can be used to move domainwalls and so change the magnetization direction of regions of the trackeven under circumstances where large external magnetic fields are neededto change the magnetization direction of the track. This is because themethod of changing the local magnetization direction is very differentwhether using external magnetic fields or domain wall fringing fields.

The tracks may be lithographically fabricated so that the choice of themagnetic material forming the track will be influenced by the methodchosen to fabricate the tracks. For example, it may be advantageous toform the track from a material which can be etched either by reactiveion etching or by wet chemical etching or by ion milling. Furthermore,it may also be advantageous to form the tracks from material which doesnot suffer corrosion during the fabrication of the tracks.

The track may be comprised of a single magnetic layer but it may beadvantageous to form the track from a multiplicity of magnetic layers.For example, it may be useful for the purposes of moving the domainwalls with smaller electrical currents to vary the electricalconductivity of the track so that the conductivity is higher at the topand bottom surfaces of the track.

This will allow for an inhomogeneous current distribution across thecross-section of the track so that the current may be higher at the topand bottom surfaces of the track. For example, the domain walls mayexperience some drag from the surfaces of the track, due, for example,to roughness of these surfaces, so that having higher current densitiesat the extremities of the track may allow for a lower overall currentthrough the track to move the domain walls. Smaller currents areadvantageous because they will lower the energy required to move thedomain walls along the tracks.

The pinning of domain walls at particular locations along the track maybe effected, as shown in track 805 of the writing device 20D in FIG. 8by introducing indentations (or protuberances) 810, 820, 830 along thetrack. The indentations (or protuberances) 810, 820, 830 are placedbetween domains 835 and 840, 845 and 850, 855 and 860, respectively.

These indentations (or protuberances) 810, 820, 830 serve to fix theregions in which the domains in the track 805 reside because the energyof the domain wall depends on the width of the track. The indentations(or protuberances) 810, 820, 830 can be of any physical form thatprovides a pinning potential for the domain walls.

As shown in FIG. 8, the indentations (or protuberances) 810, 820, 830are on both sides of the writing element track 805. Alternatively, theycan be on one side or disposed on the top or the bottom of the writingdevice's track 805. The indentations (or protuberances) can also beprotuberances. The indentations (or protuberances) in FIG. 8 serve tofix the positions of the domain walls in their quiescent states whenthey are not writing.

Indentations (or protuberances) may also be placed along the track atthe writing positions as illustrated in FIG. 8 by the indentations (orprotuberances) 815 and 825. In the present illustration, indentations(or protuberances) 815, 825 are placed in close proximity to themagnetic film 10, to fix the placement of fringing fields 865, 870, 875that are used to write to the magnetic film 10. When an appropriatecurrent is applied to track 805, the fringing fields 865 and 870 (forexample) move to indentions 815 and 820, allowing precision placement ofthe fringing field. In a further embodiment, the writing device may bemade of a combination of different ferromagnetic or ferrimagneticmaterials with indentations or protuberances.

In another embodiment, no indentation is made along the tracks of thewriting device 20 at the writing positions. The writing of the magneticfilm 10 is achieved simply by moving the domain walls and theirassociated fringing fields close to the magnetic film 10 without everhaving the domain walls stationary above and below the magnetic film 10.

The local magnetic fields of the fringing fields can be very large andcan approach the magnetization of the material, 4πM. In disk drivemagnetic recording write heads, the maximum achievable field is about4πM of the magnetic disk material. Disk drive development seeks to makethe magnetization larger, making larger magnetic moments and largerfields to ensure adequate writing to the disk.

In the present writing device, the magnitude of the domain wall fringingfields is related to the magnitude of the material used in the writingdevice 20. Local fields of several thousand oersteds are achievable.Consequently, the writing device can write strongly and reliably to themagnetic film 10. The width of the writing device is the width of thedata bit written on the magnetic film 10. In one embodiment, the typicalwidth of the writing device is about 100 nm in size.

The current required to move the domain walls along the tracks willdecrease as the area of the magnetic material forming the track isdecreased. Thus, the method of writing magnetic regions using domainwall fringing fields from domain walls is advantageous for writing verysmall magnetic regions, and, advantageously, scales with scaling of thetrack widths to smaller dimensions for higher capacity memory devices.

The width of the magnetic region will be determined by the spatialvariation of the domain wall fringing field. The extent of the writtenmagnetic region will be determined by the boundary where the domain wallfringing field exceeds the magnetic switching field of the magneticmaterial comprising the magnetic film 10. Thus, this width will berelated to the physical width of the tracks forming the writing device20 but will not be exactly the same as this width.

A device similar to a magnetic tunneling junction can be used to readthe information stored on the magnetic storage film 10. A magnetictunneling junction (MTJ) has two layers of magnetic material separatedby a thin layer of insulating material that comprises a tunnelingbarrier. This tunneling barrier is typically formed from an ultra thinlayer of aluminum oxide although it can also be formed from otherinsulating or semiconducting materials such as MgO.

One magnetic layer in the MTJ is typically a hard magnetic material thatrequires a large magnetic field to change its magnetization. The othermagnetic material is typically a soft magnetic material, allowing a weakmagnetic field to change its magnetization. When a small magnetic fieldis applied to the soft magnetic material, its direction of magnetizationchanges so that the direction of the magnetization of the soft magneticlayer can be varied relative to that of the hard magnetic material.

When a fixed voltage is applied across the MTJ, the magnitude of thecurrent passed through the tunneling barrier depends on the relativemagnetic orientation of the two magnetic materials in the tunnelingjunction. Consequently, the value of the current in the tunnelingjunction indicates the direction of the magnetic moment in the softmagnetic material if the moment of the hard layer is known. Conversely,the current in the tunneling junction indicates the direction of themoment of the hard magnetic material if the direction of the moment ofthe soft magnetic material is known.

The two magnetic materials in the magnetic tunneling junction can alsobe formed from hard magnetic materials if means for independentlyswitching the magnetic moments in the MTJ are provided, as in themagnetic memory device 100. The tunneling current passing through theMTJ allows the direction of the magnetic moment of either one of the twomagnetic materials in the MTJ, i.e., the storage layer, to be determinedif the direction of the magnetic moment of the other material, i.e., thereference layer, is known.

There is a sequence of domain walls within the tracks of the writingdevice 20. In one region, the moments are in one direction. In anotherregion, the moments are in another direction. The sequence of domainwalls within the track is known to the controller of the magnetic memorydevice 100. The controller keeps track of the location of the domainwalls in the track. Consequently, the moment of the data bit 45 can bedetermined by passing a current from the upper track through the databit 45 and the lower track.

A large value of current indicates that the resistance of the MTJ islow, and the domains of the track and the data bit 45 are in parallel. Asmall value of current indicates that the resistance of the MTJ is high,and the domains of the track and the data bit 45 are anti-parallel.Since the direction of the domain in the track is known, the directionof the domain and thus the data stored in the data bit 45 can beinferred.

A method 900 for reading a data bit 45 in the magnetic film 10 by theuse of a magnetic tunnel junction is described by the process flow chartof FIG. 9 in conjunction with FIG. 10 (FIGS. 10A and 10B). As shown inFIG. 10 the magnetic tunnel junction used for the purposes of readingthe data in the magnetic storage film 10 is formed from one or both ofthe upper and lower tracks 25 and 35, respectively, in addition to thestorage film itself.

The tracks 25 and 35 are separated from the storage film at the writingpoints where the track comes close to the storage layer by thininsulating layers 1080 and 1090 which form the insulating barrier of themagnetic tunnel junction. For ease of fabrication, it is preferable thatone tunnel barrier be thicker than the other tunnel barrier. Since theresistance of a tunnel barrier increases exponentially with thethickness of the insulating tunnel barrier, it may be difficult to formtwo tunnel barriers with approximately equal resistance values.

In practice, however, the tunnel barrier resistance is extremelysensitive to the morphology of the tunnel barrier layer so that evenwhen tunnel barriers have nominally the same thickness, their resistancecan vary significantly. Since the morphology of the tunnel barrierdepends critically on the structure and morphology and chemicalcomposition of the underlayer on which it is grown, the morphology andthe consequent resistance of the tunnel barrier 1090, which is grown ontop of the track 35, may be different from that of the tunnel barrier1080, which is deposited on top of the magnetic storage film 10.

For the purpose of illustration, the tunnel barrier 1080 is shown asbeing thicker than the tunnel barrier 1090 in FIG. 10, so that theresistance of the tunnel junction formed between the lower magnetictrack 35 and the upper magnetic track 25 will be dominated by thethicker tunnel barrier 1080. Thus, the ferromagnetic electrodes of themagnetic tunnel junction are formed from the magnetic material of theupper track 25 and the magnetic material in the magnetic storage film 10since the resistance of the tunnel junction is determined by that of theupper insulating layer 1080. Therefore, in this example, the resistanceof the lower insulating layer 1090 is sufficiently small that it doesnot significantly contribute to the resistance of the tunnel junction.

The magnetic direction of the data bit 45, which is represented by theleft directed arrow 1055 in FIG. 10A and the right directed arrow 1065in FIG. 10B, is read as a left or right directed magnetic moment, asshown by the block 905 in FIG. 9, by passing a small current through thecorresponding upper and lower tracks 25 and 35 at whose cross point thedata bit 45 resides. The direction of the current path is shown by theline 1052 in FIG. 10A and the line 1062 in FIG. 10B.

The current is passed along the upper track 25 through the upperinsulating layer 1080, through the data bit 45, through the lowerinsulating layer 1090, and along the lower track 35 as shown in theblock 910 in FIG. 9. In FIG. 10A the direction of the moment of the databit 45 is shown as being directed in the same direction as the magneticdomain in the upper track.

Thus, the resistance of the magnetic tunnel junction will be low and thecurrent 1050 will be large, as illustrated by block 940 in FIG. 9. Bycontrast, in FIG. 10B, the magnetization of the data bit 45 is directedalong the rightmost direction and is anti-parallel to that of themagnetic domain in the upper track so the resistance of the magnetictunnel junction will be high and the current 1060 will be small. Thus,as illustrated by block 925 in FIG. 9, the data bit is determined to bea

The current used for reading the state of the data bits is much smallerthan the current used to move domains along the tracks so the readingcurrent will not affect the state of the data bits. The actual currentused for reading will depend on the resistance of the tunnel junctionwhich is designed to be sufficiently high that the resistance of theupper and lower tracks is negligible compared to the resistance of thetunnel junction.

Similarly, the higher the resistance of the tunnel junction, the smallerwill be the current leaking into the array of tunnel junctions formedfrom the array of upper and lower writing tracks. In this embodiment allof the tunnel junctions associated with each data bit are connected inparallel with each other. The reading current is passed through the databit for no longer than is necessary to get enough signal to determinewhether the tunnel junction has a high or low resistance in order tominimize the energy needed to read one bit. For this purpose, there maybe reference resistors formed from similar tunnel junctions at the arrayboundary with which the resistance of the data bit tunnel junctions canbe compared.

Thus, in the memory system 100 described herein, the MTJ is formed fromtwo hard magnetic layers. The direction of the magnetic domain in theupper track in the writing device 20 can only be altered by passingcurrent pulses along this track. The magnetic moment direction of thedata bit 45 can similarly only be varied by being subject to the largemagnetic fringing fields provided by moving domains in the upper andlower tracks. In essence, the magnetic domain in the upper or lowertracks provides the reference magnetic moment and the data bit providesthe storage magnetic moment.

If necessary, for larger read signals, the resistance of the MTJ formedfrom the magnetic domain 1025 in the upper track 25 and the magneticdata storage bit 45 can first be read by passing the current 1050through this MTJ. Then, a current can be passed along the upper track tomove the domain walls in this track by one position. Thus, the directionof the magnetic moment in the track will be changed from being left, asshown in FIG. 10A, to being right.

The resistance of the MTJ is read a second time by passing a smallreading current through the MTJ. However, the resistance will now behigh because the direction of the domain in the upper track 25 will nowbe in the opposite direction to that of the data bit 45. Passing acurrent along the upper track along, without concurrently passing acurrent along the lower track 35, will only move the domains in theupper track without changing the magnetic state of the data bit 45.

Thus, this method of reading the data bit will give much higher signalbecause this method only requires the difference in resistance of theMTJ to be read. The state of the data bit is determined by whether theresistance of the MTJ is increased or decreased when the direction ofthe magnetization within the magnetic domain in the upper track isreversed.

The material forming the magnetic film 10 is comprised of one or moremagnetic layers. The properties of the magnetic film are chosen so thatthe magnetic film can support very small magnetic regions. Moreover,these small magnetic regions, whose size is largely determined by thespatial extent of the domain wall fringing fields, must be of asufficient size that the magnetic anisotropy of this region is greatenough to withstand thermal fluctuations, i.e., the superparamagneticeffect.

When the region is too small, thermal fluctuations contain sufficientenergy at the operating temperature of the device that the magneticmoment of the magnetic regions can overcome the energy barrier providedby the magneto-crystalline anisotropy of the magnetic material withinone magnetic region. Thus, the types of materials and structures usedfor high density magnetic thin film media in magnetic recording diskdrives are suitable.

An important difference, however, is that in the memory of the presentinvention, the materials forming the magnetic storage film 10 are notlimited to polycrystalline materials. Crystalline materials may be usedto form the magnetic film 10 wherein the crystalline materials have ananisotropy direction aligned along one direction within the magneticfilm. For example, the magnetic easy axis may be aligned along thedirection of one set of tracks (for example, those above the magneticfilm 10).

In magnetic recording disk drives the magnetic bits are written aroundthe circumference of the rotating disk. Thus, the magnetic crystallineanisotropy must be aligned either randomly within the plane of themagnetic media or the anisotropy direction must be aligned either alongthe disk radius or must be tangential to the disk radius. This precludesthe use of crystalline magnetic films with a well defined crystallineaxis along one given direction.

It should be understood that the present invention is not limited tomagnetic materials whose moments are in the plane of the magnetic film10, but the moments of the magnetic films 10 may be orientedperpendicular to the plane of the film 10 since the domain wall fringingfields from the domain walls in the tracks of the writing device 20 canhave a significant component of magnetic field in a direction normal tothe surface of the magnetic wires comprising the tracks. Thus, thematerial comprising the layer 10 may be designed to have perpendicularmagnetic anisotropy, and can be crystalline or polycrystalline.

Alternate embodiments of the construction of the magnetic film 10 areillustrated by FIG. 11 (FIGS. 11A, 11B, 11C). Magnetic film 10A isconstructed using a film of one magnetic alloy.

Magnetic film 10B is a granular magnetic alloy comprised of magneticregions in an insulating film. This structure may be useful to decreasethe current that flows along the magnetic storage film 10 when readingthe data storage regions. The resistance in the plane of the film willbe much larger than the resistance perpendicular through the film sincethe film is very thin. A metallic granular film could also be used. Forexample, this may be advantageous to form a magnetic material withhigher magnetic anisotropy, since the surface of magnetic grains oftenhas enhanced magnetic anisotropy.

For improving the read performance, it may be particularly advantageousto form the magnetic film 10 from a material that has a much lowerresistance perpendicular to the layer than in the plane of the layer.For example, it may be preferred to form the magnetic film 10B from agranular material comprised of magnetic columnar grains that extend fromthe lower to the upper surfaces of the film 10B but that areelectrically isolated from one another in the direction parallel to theplane of the film 10B.

For example, these columnar magnetic grains may be isolated from eachother in the lateral direction by an insulating oxide, such as aluminumoxide or silicon oxide or magnesium oxide, or by an insulating nitride,such as aluminum nitride. Thus, the insulating material is along thegrain boundaries of the cylindrically shaped grains where the axis ofthe cylinder of these grains is oriented perpendicular to the surface ofthe magnetic film 10B.

Magnetic film 10C is an artificial antiferromagnetic structureengineered for improved stability against thermal fluctuations. Forexample, the film structure may be comprised of two ferromagnetic layers1115 and 1120 separated by an antiferromagnetic coupling layer 1110. Theantiferromagnetic coupling layer 1110 can be formed from Ru or Ru—Osalloys as described in U.S. Pat. Nos. 5,465,185 and 6,153,320 or with Crand various other non-ferromagnetic metals as described in U.S. Pat. No.5,341,118.

In all the embodiments shown in FIG. 11, the magnetic layers may beformed from multiple magnetic and non-magnetic layers. For the maximumsignal for reading the data bits it may be advantageous to form theupper and lower surfaces of the magnetic film from magnetic materialwhich, in conjunction with a suitable insulating barrier layer, givesrise to highly spin polarized tunneling current. As an example, MgObarriers may be used in conjunction with Co—Fe alloys. Thus, the signalfor reading the direction of the magnetic data bits will be maximized.

As shown in FIG. 12, a reservoir for storing bits in the tracks of thewriting device 20 is desired for optimal performance. The reservoir maybe of varying sizes, from one domain to enough length to accommodate allthe domains in the data storage region on both sides of the data storageregion.

In FIG. 12, a reservoir is shown on each side of the data storage region1205: reservoirs 1210, 1215, 1220, and 1225. Each reservoir is longenough to accommodate all the domains in the tracks that write to andread from the data storage region 1205. The domains in the tracks thatwrite to and read from the data storage region 1205 can be moved in andout of their respective reservoirs 1210, 1215, 1220, 1225 depending onthe sequence of “1”s and “0”s to be written.

In an alternative embodiment for the magnetic memory device 100, amagnetic memory device 100A is shown in FIG. 13 comprised of individualregisters 1305, 1310 that write individual data bits 1315, 1320. Eachregister represented by register 1305 is comprised of current leads1325, 1330 and a magnetic wire or track 1335.

The track 1335 is comprised of two domains 1340, 1345 and one domainwall 1350. The fringing field 1355 emanating from domain wall 1345 issufficiently large to write to data bit 1315. The current leads 1325,1330 are made of magnetic material and are of sufficient length to actas reservoirs for the domains 1340, 1345. Current 1360 is applied to theregister 1305, moving fringing field 1355 into the region of the databit 1315.

As discussed previously, the fringing field 1355 writes to the data bit1315. The current 1360 then moves the fringing field 1355 out of theregion of the data bit 1315 once the data bit 1315 has been written.Each of the registers 1305, 1310 is controlled individually by atransistor and control gate denoted by the reference 1375, that passcurrent through the register 1305, 1310 to write data to the magneticfilm 10.

The current leads 1325, 1330 and the individual write registers 1305,1310 are shown in FIG. 13 above the magnetic storage layer 10. In orderto connect these leads to individual transistors 1375 the transistorsare preferably formed in a silicon substrate beneath the magnetic film10. Thus, if the current leads are above the magnetic layer 10, vias areprovided through the magnetic film to allow connections between thesecurrent leads and the transistor switches in the silicon substrate.

As described with respect to the embodiment shown in FIG. 1 the magneticfilm 10 does not have to be continuous but can be formed from multiplemagnetic sections (each larger than an individual magnetic data bit).Thus, vias can be formed between these magnetic sections through theplane defined by the magnetic film 10. The magnetic film 10 arediscontinuous and are comprised of multiple magnetic sections separatedfrom conducting vias placed between these sections. The conducting viasare electrically isolated from the magnetic sections by insulatingmaterial.

The current leads 1325, 1330 and the individual write registers 1305,1310 can also be disposed beneath the magnetic film 10 which makes itmore straightforward to connect these leads to switching transistors ina semiconducting substrate beneath the layer.

Although the individual write registers are shown in FIG. 13 without anynotches or means of fixing the position of the domain walls in thesewrite registers, the registers can be formed with one or more notches orprotuberances or can be formed from multiple connected magnetic regionscomprised of different magnetic materials for the purpose of fixingdomain walls at the boundaries between these magnetic materials forwithin these magnetic regions. Depending on the properties of thesedifferent magnetic materials, the domain walls will have a preference tobe located either at the boundaries between these materials or away fromthese boundaries with the magnetic regions themselves.

In a preferred embodiment the individual write registers are comprisedof three magnetic domains with two magnetic domain walls which givesrise to magnetic domain fringing fields of opposite sign. Thus, bymoving one or the other of these domain walls over the magnetic regionto be written, the magnetic data bit can be written in one direction orthe opposite direction. The write register is preferably long enough tocontain a storage reservoir for these domain walls to the left and rightof the magnetic data region or in the vertical portions of the currentleads themselves to allow for greater data storage densities.

In the alternative embodiment shown in FIG. 13 in which the writeregisters only contain one domain wall, the domain wall fringing fieldcan be along one direction on one side of the domain wall and along theopposite direction on the other side of the domain wall, as illustratedin FIG. 3B for the x component of the fringing field associated with theNeel type of domain wall shown in FIG. 3A. Thus, when the domain wallshown in FIG. 3A is moved over the magnetic film 10, the magnetic databit 45 is written in one direction when the domain wall is moved fromthe left to the right along the x axis. As shown in FIG. 3B, themagnetic film is written in the negative x direction, i.e., itsmagnetization is pointing to the left.

However, if the same domain wall is moved from the right to the left,the magnetic data bit in the film 10 is written with its magnetizationdirection pointing in the positive x direction (i.e., magnetizationpoints to the right). Thus, a single domain wall can be used to writedata bits to both the left and to the right. The direction of motion ofthe domain wall (left to right or right to left) determines thedirection of writing. Thus, depending on the position of the domain wallin the write register, it may be necessary to first move the domain wallto the left or to the right in order to write a magnetic data bit in theleft direction or the right direction.

In the device shown in FIG. 13 the magnetic data bits are read bypassing small currents through one of the leads 1325, 1330 and through atunnel barrier, formed as a thin insulating layer 15 on top of themagnetic film 10, and into the magnetic data bit within the magneticfilm 10 itself. The current path can be closed either through electricalcontacts on the edges of the magnetic film 10 or on the opposite side ofthe magnetic film 10.

In an embodiment of the magnetic memory device 100A shown in FIG. 14(FIGS. 14A, 14B), magnetic memory device 100B has individual writingregisters (represented by registers 1405, 1410) situated on top of themagnetic film 10 in addition to writing registers (represented byregisters 1415, 1420) on the bottom of the magnetic film 10. Data bitssuch as data bits 1425, 1430, 1435, 1440 are staggered to increase thedata bit density and to avoid interference from neighboring fringingfields. In this embodiment, the aerial density of data bits is increasedby a factor of two over the density of magnetic memory device 100A.

In an embodiment of the magnetic memory device 100B, magnetic memorydevice 100C has write registers 1505, 1510 perpendicular to writeregisters 1515, 1520. The relative placement of the registers 1505,1510, 1515, and 1520 can be in any configuration as desired to achievethe desired aerial density and still retain the ability to apply currentto the registers.

To operate the magnetic memory device 100, the circuitry comprises, inaddition to the reading and writing elements, logic and other circuitryfor a variety of purposes, including the operation of the reading andwriting devices, the provision of current pulses to move the domainswithin the write registers, the means of coding and decoding data indata bits, etc. In one embodiment, the control circuit is fabricatedusing CMOS processes on a silicon wafer. The circuitry related to theindividual magnetic data bits will be designed to have a small footprinton the silicon wafer so as to maximize the storage capacity of thememory device while utilizing the smallest area of silicon to keep thelowest possible cost.

It is to be understood that the specific embodiments of the inventionthat have been described are merely illustrative of certain applicationof the principle of the present invention. Numerous modifications may bemade to the system and method for storing data in an unpatterned,continuous magnetic layer invention described herein without departingfrom the spirit and scope of the present invention.

1. A data storage device comprising: a unpatterned magnetic filmcomprising data regions for storing data; a track disposed in proximityto the magnetic film, wherein the track selectively defines a shiftablemagnetic domain wall; and wherein a data bit is selectively stored inone of the data regions of the magnetic film, using a fringing field ofthe magnetic domain wall in the track to selectively change a directionof a magnetic moment in the data region.